Various belt type continuously variable transmissions have been proposed as transmissions for use in vehicles like automobiles. Generally, a conventional belt type continuously variable transmission comprises two pulleys and a belt. One of the pulleys, each having a circumferentially extending V-shaped groove, is provided on a rotational shaft and the other is provided on another shaft, and the belt having a V-shaped cross section is disposed around these pulleys for power transmission. While the power is being transmitted from one of the rotational shafts to the other, the widths of the circumferential grooves of these pulleys are controlled in an inverse proportion continuously to vary in adjusting the speed change ratio of the transmission.
For example, a prior-art belt, which is used in such a belt type continuously variable transmission, comprises laminated endless flat rings and a plurality of elements, which are in contact one after another in succession along the rings, which support the elements slidably. FIG. 3 shows an example of element, which is used for constituting such a belt for use in a continuously variable transmission.
The prior-art element 200 shown in FIG. 3, which is punched out from a metal plate (not shown), comprises a body portion 201, a head portion 202 and a neck portion 203 in a one-piece body. The body portion 201 is designed to come into contact with a pulley of a continuously variable transmission (not shown), and the head portion 202 is located above the body portion 201 with the neck portion 203 connecting the body portion 201 and the head portion 202. The neck portion 203 has a width narrower than those of the head and body portions 201 and 202. Furthermore, a pair of saddle portions 204 are provided symmetrically at the right and left upper parts of the body portion 201, and a pair of ear portions 205 are provided at the right and left sides of the head portion 202 that face the saddle portions 204, respectively.
By this design of the element 200, a ring-accommodating space 210, which is to accommodate a ring (not shown), is defined on each side of the element by the upper end of the saddle portion 204, the side end of the neck portion 203 and the lower end of the ear portion 205. When rings are accommodated in the ring-accommodating spaces 210, the rings are placed on the saddle portions 204, respectively. Furthermore, a cylindrical nose portion 206 is provided extruding from one face of the head portion 202 while a cylindrical hole 207 is provided on the other face of the head portion 202, so that the nose portion 206 of one element 200 can be fitted into the hole 207 of another element 200. Moreover, a first recess 208 is provided at each upper innermost part of the saddle portions 204 in an arc figure connecting smoothly to a corresponding lower side end of the neck portion 203 while a second recess 209 is provided at each lower innermost part of the ear portions 205 in an arc figure connecting smoothly to a corresponding upper side end of the neck portions 203.
In such an element exemplified by the above description, for avoiding stress concentration at the part where the upper innermost part of the saddle portion meets the lower side end of the neck portion, for example, Japanese Utility-Model Publication No. H05(1993)-14028 discloses a method in which the bottom of the recess (for example, the first recess 208 shown in FIG. 3) at the part where the upper innermost part of the saddle portion meets the lower side end of the neck portion is in a convex figure whose peak is approximately at the center in the thickness direction of the plate forming the element.
By the way, a relatively large stress is generated in the part of the element where the lower innermost part of the ear portion meets the upper side end of the neck portion when each element comes into contact with the pulley or goes off from the pulley of a continuously variable transmission in operation. Therefore, for example, if the thickness of the element is increased to secure a sufficient strength for this part, then the efficiency of power transmission may decrease or a noise may arise. On the other hand, for example, if the above method (disclosed in Japanese Utility-Model Publication No. H05(1993)-14028) is applied also to the part where the lower innermost part of the ear portion meets the upper side end of the neck portion, then a grinding process must be added in the manufacturing work, and this process can increase the manufacturing cost of the element.